The present invention relates to magnetic particles containing iron as the main component and, more particularly, to magnetic particles which have a good affinity for a vinyl aromatic resin, an acrylic resin and a copolymer of monomers thereof generally used for a magnetic toner, and an excellent mixing property with these resins for a magnetic toner, and process for producing the same.
A developing method using composite particles obtained by dispersing magnetic particles such as magnetite particles into a resin as a developer without using a carrier, which are generally called a one-component type magnetic toner, is conventionally known and used as one of a method of developing an electrostatic latent image.
With the recent development of copying machines having a higher performance such as a capability of continuously copying a higher-quality image at a higher speed, and magnetic toner as a developer is strongly required to improve the properties and for this purpose, magnetic particles which have an excellent mixing property with a magnetic toner resin are strongly demanded.
This fact is described in Japanese Patent Application Laid-Open (KOKAI) No. 55-65406 (1980) as "Generally, magnetic particles for a magnetic toner of such as one-component type are required to have the following properties. . . . VII) To have an excellent mixing property with a resin. Ordinarily, the particle diameter of a toner is not more than several ten .mu.m, and the microscopic degree of mixing in the toner is an important property of the toner. . . . ".
Magnetite particles having an isotropic shape such as an octahedron and a sphere are mainly used as magnetic particles for a magnetic toner, which are added to a vinyl aromatic resin such as styrene resin and vinyl toluene resin, an acrylic resin such as acrylic acid resin and a methacrylic acid resin, and a copolymer of these monomers thereof, which are used as resins for a magnetic toner.
Magnetic particles which have an excellent of mixing property with a resin are in the strongest demand at present, but known magnetic particles have a poor affinity for a resin. No magnetic particles which have an excellent mixing property with a resin has been provided yet.
The affinity of particles for a resin in the present invention means the degree of the affinity of the surfaces of magnetic particle for a resin.
As an index of the dispersibility of magnetic particles in a resin, the gloss of the surfaces of a resin molding containing the magnetic particles is generally measured, and as the value is higher, the dispersibility is regarded as better.
When the present inventor measured the gloss of a resin molding made of known magnetic particles at an incident angle of 60.degree., most of them were not less than 90%, but when the incident angle was lowered to 20.degree. so as to sense minute projections and dents on the surfaces of the resin molding, the gloss of all the resin molding were reduced to not more than 90%.
The present inventor presumed that this phenomenon was caused because the known magnetic particles were present in the resin in the form of agglomerates due to the poor affinity of each particle for the resin, and that if each particle has a superior affinity for the resin, the magnetic particles have an excellent mixing property with the resin so that they are uniformly dispersed in the resin, resulting in a smooth surface of the resin molding, thereby obtaining not less than 90% of gloss even at an incident angle of 20.degree..
That is, it is considered that the gloss measured at an incident angle of 20.degree. is an index of the affinity for a resin and the magnetic particles in a resin molding the surface of which has a gloss of not less than 90% are particles having a good affinity for a resin.
As a result of studies undertaken by the present inventors so as to obtain magnetic particles having a good affinity for a resin, it has been found that by kneading, smearing and spatula-stroking magnetic particles containing iron as the main component which have an average particle diameter of 0.1 to 3.0 .mu.m by means of a wheel-type mill or an attrition mill so as to release from an agglomeration thereof, the obtained magnetic particles containing iron as the main component have an average particle diameter of 0.1 to 3.0 .mu.m and a liquid absorption of not more than 18 ml, and a gloss of the surface of a resin molding containing such magnetic particles is not less than 90% when measured at an incident angle of 20.degree.: The present invention has been achieved on the basis of this finding.